This invention relates to apparatus for supporting a workpiece, e.g., a semiconductor wafer, in a precision processing system, such as a focused ion beam system or an electron microscope system. Focused ion beam (FIB) systems can both image and etch, e.g., micromachine, a workpiece. Existing FIB systems commonly support a workpiece on fixed, rigid workpiece contact elements.
Unless firmly clamped, workpieces such as wafers held in an FIB machine or electron microscope tend to vibrate in the spans between the rigid workpiece contact elements. This is a particular problem in the acoustic environment of wafer fabrication plants. Wafer vibration can cause image degradation or produce defects during micromachining.
In addition, wafers are often somewhat curved. Having a curved wafer increases focusing problems for a precision processing system. Under atmospheric conditions, a precision processing system can firmly clamp a wafer by drawing a partial vacuum underneath the wafer. The reduced pressure pulls the wafer into contact with a backing plate, with atmospheric pressure acting over the entire wafer surface. Firm clamping via partial vacuum reduces wafer curvature, as the wafer contacts the backing plate in numerous locations. Further, the largest unsupported span of the wafer is short and possible vibration frequencies for the wafer are high.
Often precision processing systems, such as an FIB system, place a wafer in a vacuum for processing. In vacuum, the above-described partial vacuum clamping method is not available. Precision processing systems can use electrostatic forces as a substitute for the partial vacuum clamp. As with a partial vacuum clamp, electrostatic forces can act over the entire wafer surface. However, there are significant disadvantages. The equipment to achieve such electrostatic forces is expensive and generally does not produce forces of high magnitude. High voltage breakdown can occur and damage the wafer. The resulting electrostatic field can undesirably deflect the ion or electron beam. In addition, a system using electrostatic forces can have difficulty releasing the wafer when the source for generating the electrostatic forces is turned off.
Precision processing systems can apply mechanical clamping as a substitute for electrostatic forces. However, applying mechanical clamping to the wafer surface anywhere but at the edge of the wafer can occlude and damage the work area. In addition, mechanical clamping at the edge of the wafer does not ensure that the wafer, which is usually somewhat curved or otherwise not absolutely planar, will contact a substantially flat backing plate anywhere except at the edges or other fragmentary portions of the wafer. The backing plate can be designed with a greater curvature than the largest curvature possible in a wafer. However, having a curved backing plate and a correspondingly curved wafer increases focusing problems for a precision processing instrument.
It is an object of the invention to provide apparatus for supporting a workpiece in a precision FIB system.
It is another object of the invention to provide workpiece supporting apparatus that reduces workpiece vibration.
It is another object of the invention to provide workpiece supporting apparatus that is relatively easy and inexpensive to manufacture and to repair.
It is another object of the invention to provide workpiece supporting apparatus that operates effectively in a vacuum. Other objects of the invention will in part be obvious and will in part appear hereinafter.